Particle size and particle size distribution (PSD) are dominant factors governing NCM cathode powder flowability. Finer particles generally show poorer flow performance, while particle gradation (span, fine/coarse particle ratio) also strongly affects inter-particle friction, adhesion and powder bulk behaviour. This technical summary is built on JACAN dry grinding and classification experience from ncm-mill.com.
1. Effect of primary particle size
As particle size decreases, the specific surface area increases sharply. More surface contact points between particles raise van der Waals force, static attraction and surface adhesion.
- Coarser NCM particles: Lower surface area, weaker inter-particle adhesion. Particles slide past each other easily, delivering better powder flowability. But excessively large particles reduce tap density and may puncture the coating during electrode calendaring.
- Ultra-fine NCM fines (<1 μm): High specific surface area. Inter-particle attractive forces become far greater than particle gravity. Fines tend to agglomerate, bridge in hoppers and stick to pipeline walls, resulting in poor flow. Over-grinding generates excessive fines and is the most common cause of deteriorated NCM powder flow.
2. Influence of particle size distribution (span and gradation)
Flowability is not determined by D50 alone; the full PSD curve matters.
- Narrow PSD (low span): Particles have similar sizes. Void space between particles is relatively large. Less interlocking occurs, and powder flows more consistently.
- Wide PSD (high span): A mix of coarse particles and abundant fines. Small fines fill the gaps between large particles. This improves tap density, but too many fines increase adhesion and static. If the fine fraction exceeds the optimal proportion, flowability drops significantly.
For NCM, a balanced gradation with a controlled small fine fraction can achieve both acceptable tap density and good flow. Too many ultra-fines will sacrifice flow, even if tap density is high.
3. Interaction with other factors
The size-flow relationship does not act in isolation.
- Fine NCM particles are more sensitive to moisture and residual lithium. Trace moisture activates surface lithium salts, making fine particles sticky and further worsening flow.
- Fines accumulate static charges more readily during pneumatic conveying and grinding. Static attraction reinforces agglomeration.
- Particle morphology also couples with size: irregular fine particles interlock more severely than spherical particles of the same size.
Practical guidance for NCM grinding
When tuning the air classifier mill or jet mill line:
- Avoid over-grinding to suppress excessive submicron fines.
- Use precision air classification to trim the ultra-fine tail and maintain a reasonable span.
- Keep D50 in the target range while limiting the volume percentage of <1 μm particles.
In general, larger particle size improves flowability, while finer size reduces flowability. However, particle size distribution (fine fraction and span) plays an equally important role. For battery-grade NCM, the goal is not simply to make particles coarser, but to balance D50, PSD gradation, tap density and flowability by controlling the ultra-fine fraction during dry de-agglomeration and classification.
